1. Field of the Invention
The present invention relates to a roller, e.g., for smoothing paper webs. The roller includes a hard roller core, which includes metal, and an elastic coating layer located on an outer side of the hard roller core. The elastic coating layer includes an elastic matrix material and fillers embedded in the matrix material. Furthermore, the invention relates to a process for producing such a roller.
2. Discussion of Background Information
Elastic rollers of this kind are, for example, used in glazing of paper webs. In this context, an elastic roller and a hard roller, respectively, together form a pressing nip through which the paper web to be processed is guided. While the hard roller has a very smooth surface that includes, e.g., steel or chilled iron and is provided for smoothing the facing side of the paper web, the elastic roller acting on the opposite side of the paper web increases uniformity and effects compression of the paper web in the pressing nip. The size range of the rollers lies within lengths of about 3 to 12 m and diameters of about 450 to 1500 mm, respectively. They withstand linear forces of up to about 600 N/mm and compressive strain of up to about 130 N/mm2.
Since the trend in paper manufacture is to perform the glazing in online operation, i.e., the paper web leaving the paper machine or the coating machine is directly guided through the paper smoothing device (calender), the requirements for  the rollers of the smoothing device are higher than before, e.g., with respect to temperature resistance. Because of the high transport speeds of the paper web which are required for online operation and, thus, the resulting high rotation speeds of the calender rollers, the nip frequency, i.e., the frequency with which the coating is compressed and released again, is increased. As a result, roller temperatures increase. These high temperatures resulting from online operation lead to problems which, in the known elastic rollers, can ultimately lead to the destruction of the plastic coating. In particular, in known plastic coatings, maximum temperature differentials of approximately 20° C. over the width of the rollers are acceptable and the plastic materials conventionally used for the coating have a substantially higher thermal expansion coefficient than the conventionally employed steel rollers or chilled iron rollers. Therefore, a temperature increase causes high axial stress between the steel or chilled iron roller and the plastic coating connected thereto.
Because of these high stresses especially in combination with localized heat points within the plastic coating, so-called “hot spots” can occur at locations where detachment or even rupture of the plastic layer will occur.
These hot spots occur especially when in addition to the mechanical stresses and the relatively high temperature crystallization points in the form of, for example, faulty adhesive connections, deposits, or above average indentations of the elastic coating, e.g., by folds or foreign bodies on the paper web, are present. In these cases the temperature at these crystallization points of conventionally 80° C. to 90° C. can increase to 150° C. resulting in the aforementioned destruction of the plastic layer.
For controlling the properties of the elastic coating layer, fillers and/or fibers are introduced into the matrix material. Depending on the quantity and physical properties of these fillers or fibers, the physical properties of the elastic coating layer are dominated or affected by the fillers or the fibers. 